All of the earth's conventional power sources such as oil, coal, and running water are originally derived from the sun. For many years attempts have been made to use the heat of the sun directly for power and domestic needs. Outside the earth's atmosphere, an average 1350 watts per square meter is available. Under good conditions, such as a cloudless desert, approximately 1000 watts per square meter is available at the earth's surface.
Solar energy conversion systems described in the prior art are generally either so complex as to require enormous capital investment, making their utilization virtually impractical; or are so unsophisticated that their efficiency in absorbing and converting solar energy is too low to make their application practical. In the first extreme, there are numerous prior art disclosures of complicated heat storage systems utilizing state changes in various salts and other materials, many of which are extremely caustic in one state or the other, and all of which are subject to significant losses through the fundamental inefficiency of heat transfer apparatus used for transferring heat absorbed by collection units to the storage materials. Other costly aspects of complicated prior art systems include the use of multiple heliostats for redirecting the sun's rays, each of which must be individually controlled in order to track the relative motion of the sun and the earth's surface, the use of critical reflective or refractive surfaces often in shapes and forms which are extremely expensive to manufacture, and through the use of exotic materials which substantially increase the capital investment required for the collection of solar energy. Another example of the first extreme has been the use of solar power in solar furnaces. These are very large arrays of optical elements which concentrate sunlight into a small area producing very high temperatures at the focus. These elements are often unique, very large and expensive installations, with only a few operating worldwide. They are principally used for materials experiments and limited production of ceramic and abrasive materials, which cannot be readily produced in any other way. A typical working temperature for a solar furnace is 3,000.degree. K. This first extreme also includes the direct generation of electricity from solar radiation falling on solar cells such as are widely used in the space program. Because each cell can produce only low voltage and current levels, a great number of cells are needed to produce substantial amounts of electrical energy. Each cell requires two individual electrical connections and labor costs for millions of connections become prohibitive except for critical uses in inaccessible places. There are also inherent problems in transmitting low voltage direct current, and storage of large amounts of power in batteries is not presently economical.
In the other extreme, most relatively simple or unsophisticated collectors are flat plate collectors which expose large flat heat absorbing surfaces of the sun's rays. These collectors are extremely inefficient in that a substantial portion of the collected energy is reradiated, the surface which forms an efficient collector also forming an efficient radiator. Substantial heat is also lost through convection to the atmosphere. In addition, such systems, by definition, must operate at relatively low temperatures which make the storage of heat extremely expensive, since an enormous bulk of low temperature storage must be provided. Nonetheless, the main current thrust of solar energy research an development programs is directed toward production of low grade power at relatively low temperatures for domestic water and space heating. There is also considerable interest in solar powered air conditioning using the Servel process. Extensive commercial use of low grade solar power awaits the development of efficient and economical collectors so that vast areas are not needed to collect sufficient power. The basic technical problem is the development of an economical material which will absorb sunlight with an effective temperature of 6,000.degree. K. (10,000.degree. F. and not reradiate at a working temperature of around 350.degree. K. (170.degree. F.) to typical surroundings at 300.degree. K. (80.degree. F.).
In summary large scale utilization of solar energy for replacing energy presently produced by depleting natural resources such as fossil fuels has not been practical in the past since no system has been produced which combined efficiency and economy, so that both capital expense and operating expenses could be maintained at a sufficiently low level that the solar energy produced could compete economically with fuel burning systems.